[oddb]

Hi
I use a pentax k30 and this lens sigma 17-70mm .
I use in 90% of all cases 17mm,or zoom in from about 24mm to 50mm.
I only use it for landscape photos. The closest object is usually at a distanse of 5 meters.
I read somewhere about back or front focusing errors in autofocus.
Is this something I or any of you Pentaxians scould worry about?
Have anyone experienced this with this lens?
If I use live view and manual focus does this matter?
Happy new year
oddb

[pschlute]

I read somewhere about back or front focusing errors in autofocus.
Is this something I or any of you Pentaxians scould worry about?

Only if your images do not show the plane of sharpest focus to be behind or in front of where you expect it to be.

If I use live view and manual focus does this matter?

Yes. Front/back focus only affects the PDAF autofocus system, which is what you use when using the viewfinder. In LV it is a separate system and FF/BF does not occur.

[mlag]

Focus errors are best visible with max aperture (like f2.8@17mm or f4@70mm) and shorter object distances , because the sharp zone around focus point or depth of field (dof) is then minimal.

If you focus on something, do you often have the impression the things in front or behind your focus point are sharper? If it is consistently behind or in front of what you focus this could be front or backfocus...
Note that front or backfocus can depend on both camera & lens...
This can be adjusted per lens (or all lenses) with menu AF adjustment in the custom 4 menu of k30.

If it is random front back then you have to improve focussing skills, by using focus zone (point , center or full zone) depending on subject. Point focus in landscape might be needed if you want to focus on a specific object in your setting and the object is off center: if you use full zone focus, the camera autofocus might judge the setting different and focus on something closer/far than what you find important in the scene, creating more random focus errors.

If you do wide landscape, it could be less important, as you want to maximise depth of field using f8 or f11.... aperture, keeping foreground to infinity sharp.

It might matter more in portraits and object photography , where you want to blur the background using 17mmf2,8 or 70mmf4 settings. As resulting dof is small, a deviation from focus point risks to become visible (unsharp).


There are some good dof calculators online...to find out the sharp focus zone ranges.

For instance at 5 meter distance 17mmf2.8 it should hardly matter (dof range is is from 2,5 m to infinity),
compare at 70mmf4 it is only sharp from 4,5 to 5,5 meter....(dof range is only 1 meter, half 50cm in front and half behind focus point, if this is shifted +/-25 cm, you might note it => front or backfocus)
Note that other distances than 5 meter yield different results...(for 17mmf2,8 this is the optimum object distance, yields largest dof, referred as hyperfocal distance)

If your 90% use is 17mm and you keep f5.6, then everything between 1m and infinity will be reasonable sharp, front or back focus will hardly influence result...